On Wednesday, 11 July 2012 at 11:18:21 UTC, akaz wrote:
if needed, the operator !! (double exclamation mark) could be
defined.
Problem is that operator"!!" is already used asa twin
operator"!". This is shorthand for "is valid as bool":
When a type can be casted to bool, it is quicker to write "!!val"
than "cast(bool)val".
This is only moderately useful, as 90% of the time, the cast
occurs in a if/while/for, where implicit casts to bool are legal,
but still:
----
import std.stdio;
struct S
{
int v;
bool opCast() {return cast(bool)v;}
}
void foo(bool b){}
void main()
{
S s = S(5);
//bool b = s; //Error: cannot implicitly convert expression (s)
of type S to bool
bool b = !!s; //This is valid though, and shorter than //bool b
= cast(bool)s;
if(s) //But it works inside a if anyways
...
foo(!!s); //Call foo with s as boolean
}
----
I've seen this used a lot in c++. explicit casts did not exist
prior to c++11. To allow casting to bool while avoiding the
dangers of implicit casts, one design patter was to define "only"
operator"!", and use !!val as an alternative to casting to bool.
After you've seen it a few times, it feels natural, as if it was
an operator of itself.
I wouldn't be surprised if this is happening in D, so I don't
think "!!" can be taken for anything.
